Fig. 1: Circuit diagram and timing diagram of the proposed memristor-based bio-to-electrical transducer with threshold-sensing functionality.
a A circuit diagram of the transducer, consisting of a sensor electrode, two field effect transistors (FETs), a memristor with a resistance of RM, and an output resistor with a resistance of RO. A gate terminal of an upper FET is extended to the sensor electrode and exposed to an analyte solution. A lower FET provides a variable resistor with a resistance of RT. b An operation of a memristor with a single-bit property. A set voltage (VSET) (or reset voltage (VRST)) is defined as the voltage applied for the set process (or reset process), changing from the high resistance state (HRS) (or low resistance state (LRS)) to LRS (or HRS). A forming voltage (VFORM) refers to the voltage applied during the forming process that initially forms conductive filaments in the memristor. An on/off ratio is defined as the ratio between the resistance of HRS (RHRS) and the resistance of LRS (RLRS). c A timing diagram of the proposed memristor-based transducer, consisting of initialization, sensing, and reading stages. At the initialization stage, the forming and reset processes of the memristor are performed so that the RM value becomes RHRS. At the same time, an analyte solution with a biomarker concentration of C drops to the sensor electrode, and a reference voltage (VREF) is applied. As C increases, the surface potential of the extended gate (VW) increases compared to VREF. At the sensing stage, a gate voltage of the lower FET (VGS) and a drain voltage (VD) are applied. The upper FET converts from the C to a drain current (ID). An output voltage (VOUT) is formed as the product of ID and an equivalent resistance of RT, RM, and RO. When VOUT exceeds VSET, RM is converted from RHRS to RLRS. At the reading stage, VGS and VD are grounded to turn off the two FETs, and a reading voltage (VR) is applied. Consequently, VOUT exhibits either a high or low level, corresponding to RLRS or RHRS. d An equivalent circuit of the transducer at the sensing stage. e An equivalent circuit of the transducer at the reading stage.
